Various types of drill bits are employed when drilling formations in association with, for example, mining activities and oil and gas exploration. One particular type of drill bit is known as a percussion or hammer drill bit. Percussion type drill bits are positioned on the end of a drill string and engage a formation while impacting the formation and also rotating relative to the formation. While in other drill bits, the primary mode of action may be shearing of the formation due to the rotation of the drill bit, percussion drilling relies heavily on the impact mechanism for penetrating the formation. Thus as the drill bit impacts the formation and rotates relative to the formation, a borehole is formed that is approximately the same diameter as the outer radius of the drill bit. Percussion type drilling systems are often employed when a hard formation (e.g., rock) is anticipated during drilling.
Different types of systems may be used in percussion drilling. For example, one system is known as a “top hammer” system where the drill bit is placed at the end of a drill string. The drill string includes a rod coupled with the drill bit and, at an upper end of the drill string, the rod is coupled to a percussion mechanism and a rotary mechanism. In other words, the impact action and the rotational action are each provided to the drill bit from the top end of the drill string.
In another example, a down-the-hole (DTH) system (sometimes referred to as an in-the-hole system) includes a drill bit that is placed at the end of the drill string. A cylinder containing a percussion mechanism (e.g., a reciprocating piston), often referred to as a “hammer,” is coupled directly with the drill bit and positioned “down hole” during operation of the drill string. Rotation may still be imparted to the drill bit by a rotational mechanism, whether positioned at the top end of the drill string or elsewhere.
The type of drilling system being used influences the design, features and size of the drill bit. For example, the coupling mechanism used for a top hammer drill bit is conventionally a threaded coupling. The threaded coupling may include a tapered neck that provides frictional engagement between the drill bit and the rod. The entire drill bit is typically exposed within the borehole during a top hammer operation.
A DTH drill bit usually includes a splined surface (e.g., on the shank) for engagement with the cylinder/hammer mechanism enabling it to slide axially relative to the cylinder during percussion activities. A substantial portion of the drill bit (e.g., the splined shank) is conventionally disposed within the cylinder of a hammer mechanism and, thus, is not directly exposed to the formation during drilling activities due to its coupling with the cylinder.
As noted above, size may also be a feature that is at least partially determined by the type of drilling system being employed. For example, a top hammer system is typically employed for drilling of holes that are approximately 125 mm in diameter or less (the gage portion or outer diameter of the drill bit substantially corresponding with the bore hole diameter). On the other hand, DTH systems are conventionally employed for drilling holes that are greater than 125 mm in diameter.
One of the weaknesses of a top hammer type drill bit is the wear experienced by portions of the bit other than the inserts or “cutters.” As noted above, the entire drill bit body is exposed to the bore hole causing it to experience wear in features and locations other than just the cutting elements of the drill bit. For example, portions of the shank of a drill bit may experience wear, causing it to overheat and deform. In some instances, the shank may even “weld” itself to the rod that is coupled with the drill bit resulting in the loss of not only the drill bit, but the rod as well.
It is a continuous desire in the industry to provide drill bits and drilling systems having improved performance characteristics including improved wear performance, thermal characteristics and useful life.